This invention relates to aircraft braking systems, and particularly to braking systems for aircraft having two or more friction brakes.
An aircraft brake may comprise a stack of discs slidably mounted on a non-rotating torque tube surrounding a wheel axle and mounted within a landing wheel rim. Alternate (rotor) discs are drivable by the wheel rim and the remaining (stator) discs are keyed to the torque tube. Axially directed thrust is applied by an hydraulic system to squeeze the rotor and stator discs together to apply the brake.
In advanced brake systems of this kind the discs may be of carbon, normally a carbon-carbon composite in which carbon fibres are embedded in a dense carbon matrix. Manufacture of such discs is very expensive, and it is therefore highly desirable to minimize the rate of wear of the discs in service so as to reduce the frequency of disc replacement and the average cost per landing.
According to the present invention, an aircraft braking system comprising a plurality of brakes comprises means for selectively disabling at least one of said brakes for taxiing only, said means being arranged to ensure that the disabling of the brakes does not affect the directional stability of the aircraft when the braking system is operated.
Preferably in an aircraft braking system in accordance with the invention and comprising at least two brakes associated with wheels on each side of an aircraft, means is provided for disabling at least one of said brakes on each side of the aircraft during taxiing.
In a braking system comprising braked wheels on the center line of the aircraft it would be possible to disable the brakes of one or more of such wheels without affecting the directional stability of the aircraft during braking.
The invention is based on the discovery that the rate of wear of a carbon brake is dependent to a major extent on the number of braking operations effected, even though a considerable number of such operations during taxiing may dissipate relatively little energy compared with that dissipated during a landing run. By providing, in accordance with the invention, means for disabling certain brakes of the undercarriage during taxiing it is possible greatly to extend the wear life of the brakes on the aircraft.
By the above means those brakes disabled during taxiing will have a much extended life as determined by the amount of wearable material provided in the brake design and indicated by the brake wear pin detector compared with those brakes not disabled during taxiing. Means may be provided to disable different brakes selectively during a series of braking applications during taxiing so as to provide reasonably even wear on all of the brakes and thereby to make it possible to extend the disc replacement intervals for the whole of the braking system. Alternatively to extending the life of a brake by disabling it during taxiing the design may make use of this reduction in wear by reducing the amount of wearable material provided and hence reduce the weight of that brake.
In either kind of application as described in the preceding paragraph, braking during the taxiing operation is preferably under the control of an automatically-controlled system which does not require intervention by the pilot of the aircraft. Control of such a system may, for instance, be determined primarily by the wheel speed, the system being operative to disable certain brakes when the wheel speed is below a certain threshold level and above a certain minimum (to retain full braking when the aircraft is at rest). However, to avoid additional brake operation as the aircraft is about to come to rest the disabling of brakes may, preferably, be arranged to continue at speeds down to and including standstill: in this latter case manual or automatic override may be provided to enable the aircraft to be held at rest against full engine thrust. Means may be provided to respond to brake temperatures and the algorithms for determining which brakes should be disabled for taxiing may take into account the temperatures of each brake to ensure that, within limits set by other considerations,
i) the temperatures of brakes are reasonably uniform across the aircraft, or
ii) certain brakes are deliberately allowed to achieve an increased temperature (in order to take advantage of an enhanced temperaturexe2x80x94dependent friction characteristic of the brake material), or
iii) the disabled brakes are brought into operation if the temperature of one or more of the remaining brakes exceeds a predetermined level.
Alternatively the system may take into account the variation in wear between all brakes across the aircraft in order to optimize the safety and maintenance procedures for these brakes.
Preferably an over-ride capability is provided, that if a predetermined input level at the pilot""s brake pedal is exceeded all the brakes are applied as in a normal brake operating system. During the landing run, it will generally be desirable for the disabling system to be inoperative, i.e. for full braking to be provided throughout the run including any period when the wheel speed falls below the threshold level mentioned above. This may be achieved by, for example a time delay actuated by a xe2x80x98weight switchxe2x80x99 which responds to the aircraft weight being imposed on the landing gear when the aircraft touches down.